Showing 1–50 of 256 results for author: Huang, K

Spin dynamics in the van der Waals ferromagnet CrTe2 engineered by Nb doping

Authors: Dhan Raj Lawati, Prem Bahadur Karki, Jitender Kumar, Karishma Prasad, Mohamed A. Elekhtiar, Kai Huang, Bibek Tiwari, Suvechhya Lamichhane, Rupak Timalsina, Zane Hubble, John Watt, Sy-Hwang Liou, Evgeny Y. Tsymbal, Jian Wang, Kapildeb Ambal, Abdelghani Laraoui

Abstract: Understanding and controlling spin dynamics in two-dimensional (2D) van der Waals (vdW) ferromagnets is essential for their application in magnonics and hybrid quantum platforms. Here, we investigate the spin dynamics of the vdW ferromagnet 1T-CrTe_{2} and demonstrate their systematic tunability via niobium (Nb) substitution in Cr_{1-x}Nb_{x}Te_{2}(x=0-0.2). Ferromagnetic resonance (FMR) spectrosc… ▽ More Understanding and controlling spin dynamics in two-dimensional (2D) van der Waals (vdW) ferromagnets is essential for their application in magnonics and hybrid quantum platforms. Here, we investigate the spin dynamics of the vdW ferromagnet 1T-CrTe_{2} and demonstrate their systematic tunability via niobium (Nb) substitution in Cr_{1-x}Nb_{x}Te_{2}(x=0-0.2). Ferromagnetic resonance (FMR) spectroscopy reveals that Nb doping enables wide-band tuning of the resonance frequency from 40 GHz down to the few-GHz regime, accompanied by a moderate increase in the Gilbert damping constant from ~0.066 to ~0.14, while preserving robust room-temperature ferromagnetism. Complementary magnetometry shows a concurrent reduction of the Curie temperature and saturation magnetization with increasing Nb content. Density functional theory calculations attribute the observed spin-dynamic trends to Nb-induced modifications of magnetic anisotropy and magnetic exchange interactions. Furthermore, CrTe_{2} flakes (~100nm thick) exhibit lower resonance frequencies than bulk crystals, consistent with thickness-dependent magnetic anisotropy. These results establish Nb-doped CrTe_{2} as a tunable vdW ferromagnet with controllable spin dynamics, extending its functionality from spintronics to broadband magnonics and quantum magnonics. △ Less

Submitted 26 December, 2025; originally announced December 2025.

Evaluating Large Language Models in Scientific Discovery

Authors: Zhangde Song, Jieyu Lu, Yuanqi Du, Botao Yu, Thomas M. Pruyn, Yue Huang, Kehan Guo, Xiuzhe Luo, Yuanhao Qu, Yi Qu, Yinkai Wang, Haorui Wang, Jeff Guo, Jingru Gan, Parshin Shojaee, Di Luo, Andres M Bran, Gen Li, Qiyuan Zhao, Shao-Xiong Lennon Luo, Yuxuan Zhang, Xiang Zou, Wanru Zhao, Yifan F. Zhang, Wucheng Zhang , et al. (31 additional authors not shown)

Abstract: Large language models (LLMs) are increasingly applied to scientific research, yet prevailing science benchmarks probe decontextualized knowledge and overlook the iterative reasoning, hypothesis generation, and observation interpretation that drive scientific discovery. We introduce a scenario-grounded benchmark that evaluates LLMs across biology, chemistry, materials, and physics, where domain exp… ▽ More Large language models (LLMs) are increasingly applied to scientific research, yet prevailing science benchmarks probe decontextualized knowledge and overlook the iterative reasoning, hypothesis generation, and observation interpretation that drive scientific discovery. We introduce a scenario-grounded benchmark that evaluates LLMs across biology, chemistry, materials, and physics, where domain experts define research projects of genuine interest and decompose them into modular research scenarios from which vetted questions are sampled. The framework assesses models at two levels: (i) question-level accuracy on scenario-tied items and (ii) project-level performance, where models must propose testable hypotheses, design simulations or experiments, and interpret results. Applying this two-phase scientific discovery evaluation (SDE) framework to state-of-the-art LLMs reveals a consistent performance gap relative to general science benchmarks, diminishing return of scaling up model sizes and reasoning, and systematic weaknesses shared across top-tier models from different providers. Large performance variation in research scenarios leads to changing choices of the best performing model on scientific discovery projects evaluated, suggesting all current LLMs are distant to general scientific "superintelligence". Nevertheless, LLMs already demonstrate promise in a great variety of scientific discovery projects, including cases where constituent scenario scores are low, highlighting the role of guided exploration and serendipity in discovery. This SDE framework offers a reproducible benchmark for discovery-relevant evaluation of LLMs and charts practical paths to advance their development toward scientific discovery. △ Less

Submitted 17 December, 2025; originally announced December 2025.

Thermal ionization of impurity-bound quasiholes in the fractional quantum Hall effect

Authors: Ke Huang, Sankar Das Sarma, Xiao Li

Abstract: We study the interplay between a Coulomb impurity and quasiholes in a fractional quantum Hall (FQH) state at finite temperatures. While a repulsive impurity can pin a quasihole and stabilize the FQH state, an attractive impurity cannot bind quasiholes. We demonstrate that at finite temperatures, a quasihole can be thermally ionized from a repulsive impurity, resulting in an ionization phase transi… ▽ More We study the interplay between a Coulomb impurity and quasiholes in a fractional quantum Hall (FQH) state at finite temperatures. While a repulsive impurity can pin a quasihole and stabilize the FQH state, an attractive impurity cannot bind quasiholes. We demonstrate that at finite temperatures, a quasihole can be thermally ionized from a repulsive impurity, resulting in an ionization phase transition. We propose an experimental setup using exciton sensing to detect such a thermal ionization of quasiholes. △ Less

Submitted 8 December, 2025; originally announced December 2025.

Comments: 5 pages, 3 figures. Comments are welcome

A Machine Learning study of the two-dimensional antiferromagnetic $q$-state Potts model on the square lattice

Authors: Shang-Wei Li, Kai-Wei Huang, Chien-Ting Chen, Fu-Jiun Jiang

Abstract: The critical phenomena of two-dimensional (2D) antiferromagnetic $q$-state Potts model on the square lattice with $q=2,3,4,5$ and 6 are investigated using the technique of supervised neural network (NN). Unlike the conventional NN approaches, here we train a multilayer perceptron consisting of only one input layer, one hidden layer, and one output layer with two artificially made stagger-like conf… ▽ More The critical phenomena of two-dimensional (2D) antiferromagnetic $q$-state Potts model on the square lattice with $q=2,3,4,5$ and 6 are investigated using the technique of supervised neural network (NN). Unlike the conventional NN approaches, here we train a multilayer perceptron consisting of only one input layer, one hidden layer, and one output layer with two artificially made stagger-like configurations. Remarkably, despite the fact that the MLP is trained without any input from these considered models, it correctly identifies the critical temperatures of the studied physical systems. Particularly, the MLP outcomes suggest convincingly that the $q=3$ model is critical only at zero temperature and $q=4,5,6$ models remain disordered at all temperatures. Previously, this MLP has been successfully applied to uncover the nature of the phase transitions of 2D antiferromagnetic Ising model with multi-interactions. Therefore, it will be interesting to examine whether the already trained MLP can detect other models with untypical critical phenomena. △ Less

Submitted 7 December, 2025; originally announced December 2025.

Comments: 10 pages, 10 figures

A Machine Learning study of the two-dimensional antiferromagnetic Ising model with nearest and next-to-nearest interactions on the triangular lattice

Authors: Shang-Wei Li, Yuan-Heng Tseng, Kai-Wei Huang, Fu-Jiun Jiang

Abstract: We study the phase transitions of the two-dimensional antiferromagnetic Ising model with nearest $J_1$ and next-to-nearest $J_2$ interactions on the triangular lattice for $J_2/J_1 = 0.1, 0.5$ and 1.0. The method of supervised neural networks (NN) is employed for the investigation. While supervised NN is used, no real spin configurations are needed for the training. In addition, two kinds of confi… ▽ More We study the phase transitions of the two-dimensional antiferromagnetic Ising model with nearest $J_1$ and next-to-nearest $J_2$ interactions on the triangular lattice for $J_2/J_1 = 0.1, 0.5$ and 1.0. The method of supervised neural networks (NN) is employed for the investigation. While supervised NN is used, no real spin configurations are needed for the training. In addition, two kinds of configurations having their spins be arranged in a staggered pattern are considered as the training set. Remarkably, with this unconventional training strategy, not only the critical temperatures of the studied $J_2/J_1$ are computed accurately by the resulting NN, but also the nature of the investigated phase transitions are determined correctly. Specifically, the phase transitions associated with $J_2/J_1 = 0.1, 0.5$ and 1.0 are first order. These conclusions are consistent with the known results obtained by other methods. Since the training strategy is simple, the NN calculations is highly efficient. It remains to examine whether the unconventional training approach considered in this study can be used to investigate other models with untypical phase transitions or with nontrivial ground state configurations. △ Less

Submitted 17 November, 2025; originally announced November 2025.

Comments: 12 pages, 15 figures

Third-order quantum phase transitions of bosonic non-Abelian fractional quantum Hall states

Authors: Kai-Wen Huang, Xiang-Jian Hou, Ying-Hai Wu

Abstract: We study phase transitions in bilayer and trilayer bosonic quantum Hall systems. In the absence of interlayer tunneling and interaction, each layer is chosen to have filling factor $ν=1/2$ or $1$ to realize the Laughlin state or the Moore-Read state. By tuning interlayer tunneling and/or interaction, multiple phases can be generated. In the absence of interlayer interaction, three phase transition… ▽ More We study phase transitions in bilayer and trilayer bosonic quantum Hall systems. In the absence of interlayer tunneling and interaction, each layer is chosen to have filling factor $ν=1/2$ or $1$ to realize the Laughlin state or the Moore-Read state. By tuning interlayer tunneling and/or interaction, multiple phases can be generated. In the absence of interlayer interaction, three phase transitions appear when interlayer tunneling becomes sufficiently strong: (1) from two decoupled $ν=1/2$ Laughlin states to the Moore-Read state in bilayer systems; (2) from one $ν=1/2$ Laughlin state plus one $ν=1$ Moore-Read state to the Read-Rezayi $\mathbb{Z}_{3}$ state in bilayer systems; (3) from three decoupled $ν=1/2$ Laughlin states to the Read-Rezayi $\mathbb{Z}_{3}$ state in trilayer systems. Numerical calculations suggest that these transitions are third-order ones. We propose non-Abelian Chern-Simons-Higgs theory to describe them. If both interlayer tunneling and interaction are present, one-component or multi-component composite fermion liquids and Jain states can be realized. This leads to intricate phase diagrams that host multiple phase transitions and possibly exotic critical points. △ Less

Submitted 21 September, 2025; originally announced September 2025.

Comments: 10 pages, 7 figures

Mo Atom Rearrangement Drives Layer-Dependent Reactivity in Two-Dimensional MoS2

Authors: Zifan Wang, Jiaxuan Wen, Tina Mihm, Shaopeng Feng, Kelvin Huang, Jing Tang, Tianshu Li, Liangbo Liang, Sahar Sharifzadeh, Keji Lai, Xi Ling

Abstract: Two-dimensional (2D) materials offer a valuable platform for manipulating and studying chemical reactions at atomic level, owing to the ease of controlling their microscopic structure at the nanometer scale. While extensive research has been conducted on the structure-dependent chemical activity of 2D materials, the influence of structural transformation during the reaction remains largely unexplo… ▽ More Two-dimensional (2D) materials offer a valuable platform for manipulating and studying chemical reactions at atomic level, owing to the ease of controlling their microscopic structure at the nanometer scale. While extensive research has been conducted on the structure-dependent chemical activity of 2D materials, the influence of structural transformation during the reaction remains largely unexplored. In this work, we report the layer-dependent chemical reactivity of MoS2 during a nitridation atomic substitution reaction and attribute it to the rearrangement of Mo atoms. Our results show that the chemical reactivity of MoS2 decreases as the number of layers is reduced in the few-layer regime. In particular, monolayer MoS2 exhibits significantly lower reactivity compared to its few-layer and multilayer counterparts. Atomic-resolution transmission electron microscope (TEM) reveals that MoN nanonetworks form as reaction products from monolayer and bilayer MoS2, with the continuity of the MoN crystals increasing with layer number, consistent with the local conductivity mapping data. The layer-dependent reactivity is attributed to the relative stability of the hypothetically formed MoN phase which retain the number of Mo atomic layers present in the precursor. Specifically, the low chemical reactivity of monolayer MoS2 is attributed to the high energy cost associated with Mo atom diffusion and migration necessary to form multi-layer Mo lattices in the thermodynamically stable MoN phase. This study underscores the critical role of lattice rearrangement in governing chemical reactivity and highlights the potential of 2D materials as versatile platforms for advancing the understanding of materials chemistry at atomic scale. △ Less

Submitted 4 September, 2025; originally announced September 2025.

Comments: 41 pages, 20 figures

Observation and Modulation of the Quantum Mpemba Effect on a Superconducting Quantum Processor

Authors: Yueshan Xu, Cai-Ping Fang, Bing-Jie Chen, Ming-Chuan Wang, Zi-Yong Ge, Yun-Hao Shi, Yu Liu, Cheng-Lin Deng, Kui Zhao, Zheng-He Liu, Tian-Ming Li, Hao Li, Ziting Wang, Gui-Han Liang, Da'er Feng, Xueyi Guo, Xu-Yang Gu, Yang He, Hao-Tian Liu, Zheng-Yang Mei, Yongxi Xiao, Yu Yan, Yi-Han Yu, Wei-Ping Yuan, Jia-Chi Zhang , et al. (11 additional authors not shown)

Abstract: In non-equilibrium quantum many-body systems, the quantum Mpemba effect (QME) emerges as a counterintuitive phenomenon: systems exhibiting greater initial symmetry breaking restore symmetry faster than those with less. While theoretical exploration of QME has surged, experimental studies on its multidimensional modulation remain limited. Here, we report the observation and control of QME using a s… ▽ More In non-equilibrium quantum many-body systems, the quantum Mpemba effect (QME) emerges as a counterintuitive phenomenon: systems exhibiting greater initial symmetry breaking restore symmetry faster than those with less. While theoretical exploration of QME has surged, experimental studies on its multidimensional modulation remain limited. Here, we report the observation and control of QME using a superconducting processor featuring a unique fully connected, tunable-coupling architecture that enables precise modulation from short- to long-range interactions. This platform allows independent manipulation of coupling regimes, on-site potentials, and initial states, elucidating their roles in QME. To quantify symmetry restoration, we employ entanglement asymmetry (EA) -- the relative entropy between a subsystem reduced density matrix and its symmetric projection -- as a sensitive probe of symmetry breaking. In strong short-range coupling regimes, EA crossovers during quenches from tilted Néel states confirm the presence of QME. In contrast, in intermediate coupling regimes, synchronized EA and entanglement entropy dynamics reveal the suppression of QME. Remarkably, QME reemerges with the introduction of on-site linear potentials or quenches from tilted ferromagnetic states, the latter proving robust against on-site disorder. Our study provides the first demonstration of flexible QME modulation on a superconducting platform with multiple controllable parameters, shedding light on quantum many-body non-equilibrium dynamics and opening avenues for quantum information applications. △ Less

Submitted 11 August, 2025; originally announced August 2025.

Numerical extraction of crosscap coefficients in microscopic models for (2+1)D conformal field theory

Authors: Jia-Ming Dong, Yueshui Zhang, Kai-Wen Huang, Hong-Hao Tu, Ying-Hai Wu

Abstract: Conformal field theory (CFT) can be placed on disparate space-time manifolds to facilitate investigations of their properties. For (2+1)-dimensional [(2+1)D] theories, one useful choice is the real projective space $\mathbb{RP}^3$ obtained by identifying antipodal points on the boundary sphere of a three-dimensional ball. One-point functions of scalar primary fields on this manifold generally do n… ▽ More Conformal field theory (CFT) can be placed on disparate space-time manifolds to facilitate investigations of their properties. For (2+1)-dimensional [(2+1)D] theories, one useful choice is the real projective space $\mathbb{RP}^3$ obtained by identifying antipodal points on the boundary sphere of a three-dimensional ball. One-point functions of scalar primary fields on this manifold generally do not vanish and encode the so-called crosscap coefficients. These coefficients also manifest on the sphere as the overlaps between certain crosscap states and CFT primary states. Taking the (2+1)D Ising CFT as a concrete example, we demonstrate that crosscap coefficients can be extracted from microscopic models. We construct crosscap states in both lattice models defined on polyhedrons and continuum models in Landau levels, where the degrees of freedom at antipodal points are entangled in Bell-type states. By computing their overlaps with the eigenstates of many-body Hamiltonians, we obtain results consistent with those from conformal bootstrap. Importantly, our approach directly reveals the absolute values of crosscap overlaps, whereas bootstrap calculations typically yield only their ratios. Furthermore, we investigate the finite-size scaling of these overlaps and their evolution under perturbations away from criticality. △ Less

Submitted 3 December, 2025; v1 submitted 26 July, 2025; originally announced July 2025.

Comments: 7 pages, 3 figures

Journal ref: Phys. Rev. D 112, L121701 (2025)

Many-body delocalization with a two-dimensional 70-qubit superconducting quantum simulator

Authors: Tian-Ming Li, Zheng-Hang Sun, Yun-Hao Shi, Zhen-Ting Bao, Yong-Yi Wang, Jia-Chi Zhang, Yu Liu, Cheng-Lin Deng, Yi-Han Yu, Zheng-He Liu, Chi-Tong Chen, Li Li, Hao Li, Hao-Tian Liu, Si-Yun Zhou, Zhen-Yu Peng, Yan-Jun Liu, Ziting Wang, Yue-Shan Xu, Kui Zhao, Yang He, Da'er Feng, Jia-Cheng Song, Cai-Ping Fang, Junrui Deng , et al. (13 additional authors not shown)

Abstract: Quantum many-body systems with sufficiently strong disorder can exhibit a non-equilibrium phenomenon, known as the many-body localization (MBL), which is distinct from conventional thermalization. While the MBL regime has been extensively studied in one dimension, its existence in higher dimensions remains elusive, challenged by the avalanche instability. Here, using a 70-qubit two-dimensional (2D… ▽ More Quantum many-body systems with sufficiently strong disorder can exhibit a non-equilibrium phenomenon, known as the many-body localization (MBL), which is distinct from conventional thermalization. While the MBL regime has been extensively studied in one dimension, its existence in higher dimensions remains elusive, challenged by the avalanche instability. Here, using a 70-qubit two-dimensional (2D) superconducting quantum simulator, we experimentally explore the robustness of the MBL regime in controlled finite-size 2D systems. We observe that the decay of imbalance becomes more pronounced with increasing system sizes, scaling up from 21, 42 to 70 qubits, with a relatively large disorder strength, and for the first time, provide an evidence for the many-body delocalization in 2D disordered systems. Our experimental results are consistent with the avalanche theory that predicts the instability of MBL regime beyond one spatial dimension. This work establishes a scalable platform for probing high-dimensional non-equilibrium phases of matter and their finite-size effects using superconducting quantum circuits. △ Less

Submitted 22 July, 2025; originally announced July 2025.

Comments: main text: 7 pages, 3 figures; supplementary information: 19 pages, 17 figures

Weyl-Superconductivity revealed by Edge Mode mediated Nonlocal Transport

Authors: Wenyao Liu, Gabriel Natale, Camron Farhang, Michael Geiwitz, Kewen Huang, Qishuo Tan, Xingyao Guo, Mason Gray, Vincent Lamberti, Jazzmin Victorin, Huairuo Zhang, James L. Hart, Vsevolod Belosevich, Xi Ling, Qiong Ma, Wan Kyu Park, Kenji Watanabe, Takashi Taniguchi, Judy J. Cha, Albert V. Davydov, Kin Chung Fong, Ethan Arnault, Genda Gu, Rui-Xing Zhang, Enrico Rossi , et al. (2 additional authors not shown)

Abstract: Topological superconductivity (TSC) hosts exotic modes enabling error-free quantum computation and low-temperature spintronics. Despite preliminary evidence of edge modes, unambiguous signatures remain undetected. Here, we report the first observation of protected, non-local transport from the edge modes of the potential Weyl-superconductor \ch{FeTe_{0.55}Se_{0.45}}. Namely resonant charge injecti… ▽ More Topological superconductivity (TSC) hosts exotic modes enabling error-free quantum computation and low-temperature spintronics. Despite preliminary evidence of edge modes, unambiguous signatures remain undetected. Here, we report the first observation of protected, non-local transport from the edge modes of the potential Weyl-superconductor \ch{FeTe_{0.55}Se_{0.45}}. Namely resonant charge injection, ballistic transport, and extraction via edge modes. An anomalous conductance plateau emerges only when topological, superconducting, and magnetic phases coexist, with source-drain contacts coupled via the edge. Moving the drain to the bulk switches the non-local transport process to a local Andreev process, generating a zero-bias conductance peak (ZBCP). The edge mode's topological protection is confirmed by its insensitivity to external magnetic fields and increasing temperatures until the spontaneous magnetization is substantially suppressed. Our findings provide a new methodology to demonstrate TSC edge states in \ch{FeTe_{0.55}Se_{0.45}} via topologically protected non-local transport. △ Less

Submitted 1 July, 2025; originally announced July 2025.

Comments: 30 pages, 4 figures

Hetero-Orbital Two-Component Fractional Quantum Hall States in Bilayer Graphene

Authors: Ke Huang, Ajit C. Balram, Hailong Fu, Chengqi Guo, Kenji Watanabe, Takashi Taniguchi, Jainendra K. Jain, Jun Zhu

Abstract: A two-dimensional electron system exposed to a strong magnetic field produces a plethora of strongly interacting fractional quantum Hall (FQH) states, the complex topological orders of which are revealed through exotic emergent particles, such as composite fermions, fractionally charged Abelian and non-Abelian anyons. Much insight has been gained by the study of multi-component FQH states, where s… ▽ More A two-dimensional electron system exposed to a strong magnetic field produces a plethora of strongly interacting fractional quantum Hall (FQH) states, the complex topological orders of which are revealed through exotic emergent particles, such as composite fermions, fractionally charged Abelian and non-Abelian anyons. Much insight has been gained by the study of multi-component FQH states, where spin and pseudospin indices of the electron contribute additional correlation. Traditional multi-component FQH states develop in situations where the components share the same orbital states and the resulting interactions are pseudospin independent; this homo-orbital nature was also crucial to their theoretical understanding. Here, we study "hetero-orbital" two-component FQH states, in which the orbital index is part of the pseudospin, rendering the multi-component interactions strongly SU(2) anisotropic in the pseudospin space. Such states, obtained in bilayer graphene at the isospin transition between N = 0 and N = 1 electron Landau levels, are markedly different from previous homo-orbital two-component FQH states. In particular, we observe strikingly different behaviors for the parallel-flux and reverse-flux composite fermion states, and an anomalously strong two-component 2/5 state over a wide range of magnetic field before it abruptly disappears at a high field. Our findings, combined with detailed theoretical calculations, reveal the surprising robustness of the hetero-orbital FQH effects, significantly enriching our understanding of FQH physics in this novel regime. △ Less

Submitted 26 July, 2025; v1 submitted 17 June, 2025; originally announced June 2025.

Comments: 21 pages, 12 figures including appendices

Journal ref: Phys. Rev. X 15, 031023 (2025)

Towards Sustainable Energy Storage: Evaluating Polymer Electrolytes for Zinc Ion Batteries

Authors: Roya Rajabi, Shichen Sun, Booker Wu, Jamil Khan, Kevin Huang

Abstract: Polymer electrolytes present a promising solution to the challenges posed by aqueous electrolytes in energy storage systems, offering the flexibility needed for wearable electronics. Despite the increasing interest in polymer electrolyte-based zinc ion batteries (ZIBs), their development is still in its early stages due to various challenges. In this study, we fabricated three promising polymer el… ▽ More Polymer electrolytes present a promising solution to the challenges posed by aqueous electrolytes in energy storage systems, offering the flexibility needed for wearable electronics. Despite the increasing interest in polymer electrolyte-based zinc ion batteries (ZIBs), their development is still in its early stages due to various challenges. In this study, we fabricated three promising polymer electrolytes: CSAM (carboxyl methyl chitosan with acrylamide monomer), PAM (polyacrylamide monomer hydrogel electrolyte), and p-PBI (Phosphoric acid (PA)-doped polybenzimidazole) with Zn(ClO4)2 and Zn(OTf)2, for their application in zinc ion batteries. Our results demonstrated that PAM hydrogel electrolyte exhibited very low LDH formation after a long cycle, demonstrating effective protection for zinc foil, and the high mechanical stability of the p-PBI membrane provided prolonged durability against short circuits through the formation of LDH. The presence of carboxyl groups in CSAM and the formation of O-H bonding facilitated ion movement, resulting in enhanced ionic conductivity, and preventing dendrite formation. Incorporating these hydrogels with high-performance zinc salts, such as zinc triflate (Zn(OTf)2), resulted in impressive stability, with the symmetric cell demonstrating over 4000 hours of uniform and stable voltage profile under 1 mA/cm2 and low overpotential of around 53 mV cycling with CSAM. The full-cell battery with PBI-T membrane showed the highest durability and capacity compared to CSAM-T and PAM-T, due to the greater availability of free protons for storing zinc in the cathode. △ Less

Submitted 3 May, 2025; originally announced May 2025.

Parameter Sensitivity Analysis in Zinc-Ion Batteries: A Study on Ionic Conductivity, Current Density, and Electrode Properties

Authors: Roya Rajabi, Shichen Sun, Booker Wu, Jamil Khan, Kevin Huang

Abstract: This study presents a comprehensive Multiphysics model for zinc-ion batteries (ZIBs), incorporating electrochemical aspects. The model integrates the mass transport of Zn2+ ions, charge transfer, and solid diffusion to predict performance parameters like cell potential, and energy density. Significant research has focused on enhancing battery performance by optimizing components of battery to impr… ▽ More This study presents a comprehensive Multiphysics model for zinc-ion batteries (ZIBs), incorporating electrochemical aspects. The model integrates the mass transport of Zn2+ ions, charge transfer, and solid diffusion to predict performance parameters like cell potential, and energy density. Significant research has focused on enhancing battery performance by optimizing components of battery to improve parameters such as ionic conductivity and exchange current density and capacity. In this study, we present a model-based investigation of zinc-ion batteries, examining the impact of these parameters. Our findings reveal that at low current densities, raising of ionic conductivity beyond 1.3 S/m and exchange current density above 0.13 mA/cm2 do not yield substantial improvements in capacity. These insights underscore the importance of identifying performance thresholds in the development of next-generation batteries. △ Less

Submitted 3 May, 2025; originally announced May 2025.

Stacking-orientation and twist-angle control on integer and fractional Chern insulators in moiré rhombohedral graphene

Authors: Chushan Li, Chuanqi Zheng, Kai Liu, Ke Huang, Zheng Sun, Lei Qiao, Yifan Wei, Chenyu Zhang, Fan Xu, Kenji Watanabe, Takashi Taniguchi, Hao Yang, Dandan Guan, Liang Liu, Shiyong Wang, Yaoyi Li, Hao Zheng, Canhua Liu, Bingbing Tong, Li Lu, Jinfeng Jia, Zhiwen Shi, Jianpeng Liu, Xiao Li, Guorui Chen , et al. (2 additional authors not shown)

Abstract: Rhombohedral-stacked multilayer graphene aligned with hexagonal boron nitride has emerged as an excellent platform for investigating exotic quantum phenomena arising from the interplay between electron correlations and nontrivial topology. However, the microscopic mechanism governing the emergence of both the integer and fractional Chern insulator states in this system remains an open question. In… ▽ More Rhombohedral-stacked multilayer graphene aligned with hexagonal boron nitride has emerged as an excellent platform for investigating exotic quantum phenomena arising from the interplay between electron correlations and nontrivial topology. However, the microscopic mechanism governing the emergence of both the integer and fractional Chern insulator states in this system remains an open question. In this work, we systematically investigate the electrical transport properties of RMG/hBN moiré devices with controlled alignment orientations and twist angles. We demonstrate that alignment orientation strongly modulates correlated phenomena in the moiré-proximal regime, while having negligible influence on the formation of integer and fractional Chern insulators in the moiré-distant regime. Instead, the moiré periodicity, tuned by the twist angle, serves as the key parameter controlling the stability of these correlated topological states in the moiré-distant regime. Furthermore, in the moiré-proximal regime of one specific alignment, we observe anomalous Hall effect and a variety of competing phases near ν = 1, including integer Chern insulator states, extended Chern insulator states, and trivial insulators, whose stability is highly sensitive to both the applied displacement electric field and magnetic field. Our results underscore the critical role of stacking-alignment and twist-angle engineering in exploring novel quantum states based on rhombohedral-stacked multilayer graphene moiré systems. △ Less

Submitted 11 November, 2025; v1 submitted 3 May, 2025; originally announced May 2025.

Unveiling competitions between carrier recombination pathways in semiconductors via mechanical damping

Authors: Mingyu Xie, Ruitian Chen, Jiaze Wu, Kaiqi Qiu, Mingqiang Li, Huicong Chen, Kai Huang, Yu Zou

Abstract: The total rate of carrier recombination in semiconductors has conventionally been expressed using an additive model, r_total = Σr_i , which rules out the interactions between carrier recombination pathways. Here we challenge this paradigm by demonstrating pathway competitions using our newly developed light-induced mechanical absorption spectroscopy (LIMAS), which allows us to probe genuine recomb… ▽ More The total rate of carrier recombination in semiconductors has conventionally been expressed using an additive model, r_total = Σr_i , which rules out the interactions between carrier recombination pathways. Here we challenge this paradigm by demonstrating pathway competitions using our newly developed light-induced mechanical absorption spectroscopy (LIMAS), which allows us to probe genuine recombination dynamics in semiconductors via mechanical damping. We show that the total recombination rate in zinc sulfide (ZnS), a model semiconductor material, follows a multiplicative weighting model, r_total \propto Πr_i ^(w_i) with Σw_i=1. Under both steady-state and switch-on illuminations, the weighting factors w_i for each recombination pathway-direct, trap-assisted, and sublinear-are dictated by the carrier generation mechanism: (i) interband transition favors direct recombination; (ii) single-defect level-mediated generation promotes trap-assisted recombination; (iii) generation involving multiple saturated defect levels gives rise to sublinear recombination. Upon light switch-off, localized state changes drive a dynamic evolution of w_i, altering pathway competitions. These findings reshape our fundamental understanding of carrier dynamics and provide a new strategy to optimize next-generation optoelectronic devices. △ Less

Submitted 1 May, 2025; originally announced May 2025.

Microwave-activated high-fidelity three-qubit gate scheme for fixed-frequency superconducting qubits

Authors: Kui Zhao, Wei-Guo Ma, Ziting Wang, Hao Li, Kaixuan Huang, Yun-Hao Shi, Kai Xu, Heng Fan

Abstract: Scalable superconducting quantum processors require balancing critical constraints in coherence, control complexity, and spectral crowding. Fixed-frequency architectures suppress flux noise and simplify control via all-microwave operations but remain limited by residual ZZ crosstalk. Here we propose a microwave-activated three-qubit gate protocol for fixed-frequency transmon qubits in the large-de… ▽ More Scalable superconducting quantum processors require balancing critical constraints in coherence, control complexity, and spectral crowding. Fixed-frequency architectures suppress flux noise and simplify control via all-microwave operations but remain limited by residual ZZ crosstalk. Here we propose a microwave-activated three-qubit gate protocol for fixed-frequency transmon qubits in the large-detuning regime ($|Δ| \gg g$), leveraging the third-order nonlinear interaction to coherently exchange $\ket{001} \leftrightarrow \ket{110}$ states. By incorporating a phase-compensated optimization protocol, numerical simulations demonstrate a high average gate fidelity exceeding $99.9\%$. Systematic error analysis identifies static long-range ZZ coupling as the dominant error source in multi-qubit systems, which can be suppressed via operations in the large-detuning regime ($\sim 1$ GHz). The protocol maintains process fidelities exceeding $98\%$ under decoherence, while demonstrating intrinsic robustness to fabrication-induced parameter variations and compatibility with existing all-microwave two-qubit gate architectures. This hardware-efficient strategy advances scalable quantum computing systems by improving coherence properties, reducing spectral congestion, and expanding the experimental toolkit for error-resilient quantum operations in the noisy intermediate-scale quantum era. △ Less

Submitted 16 October, 2025; v1 submitted 30 April, 2025; originally announced April 2025.

Comments: 16 pages, 11 figures

Journal ref: Phys. Rev. Applied 24, 034064 (2025)

Prethermalization by Random Multipolar Driving on a 78-Qubit Superconducting Processor

Authors: Zheng-He Liu, Yu Liu, Gui-Han Liang, Cheng-Lin Deng, Keyang Chen, Yun-Hao Shi, Tian-Ming Li, Lv Zhang, Bing-Jie Chen, Cai-Ping Fang, Da'er Feng, Xu-Yang Gu, Yang He, Kaixuan Huang, Hao Li, Hao-Tian Liu, Li Li, Zheng-Yang Mei, Zhen-Yu Peng, Jia-Cheng Song, Ming-Chuan Wang, Shuai-Li Wang, Ziting Wang, Yongxi Xiao, Minke Xu , et al. (21 additional authors not shown)

Abstract: Time-dependent drives hold the promise of realizing non-equilibrium many-body phenomena that are absent in undriven systems. Yet, drive-induced heating normally destabilizes the systems, which can be parametrically suppressed in the high-frequency regime by using periodic (Floquet) drives. It remains largely unknown to what extent highly controllable quantum simulators can suppress heating in non-… ▽ More Time-dependent drives hold the promise of realizing non-equilibrium many-body phenomena that are absent in undriven systems. Yet, drive-induced heating normally destabilizes the systems, which can be parametrically suppressed in the high-frequency regime by using periodic (Floquet) drives. It remains largely unknown to what extent highly controllable quantum simulators can suppress heating in non-periodically driven systems. Using the 78-qubit superconducting quantum processor, Chuang-tzu 2.0, we report the experimental observation of long-lived prethermal phases in many-body systems with tunable heating rates, driven by structured random protocols, characterized by $n$-multipolar temporal correlations. By measuring both the particle imbalance and subsystem entanglement entropy, we monitor the entire heating process over 1,000 driving cycles and observe the existence of the prethermal plateau. The prethermal lifetime is `doubly tunable': one way by driving frequency, the other by multipolar order; it grows algebraically with the frequency with the universal scaling exponent $2n{+}1$. Using quantum state tomography on different subsystems, we demonstrate a non-uniform spatial entanglement distribution and observe a crossover from area-law to volume-law entanglement scaling. With 78 qubits and 137 couplers in a 2D configuration, the entire far-from-equilibrium heating dynamics are beyond the reach of simulation using tensor-network numerical techniques. Our work highlights superconducting quantum processors as a powerful platform for exploring universal scaling laws and non-equilibrium phases of matter in driven systems in regimes where classical simulation faces formidable challenges. △ Less

Submitted 1 April, 2025; v1 submitted 27 March, 2025; originally announced March 2025.

Two-dimensional antiferromagnets with non-relativistic spin splitting switchable by electric polarization

Authors: Himanshu Mavani, Kai Huang, Kartik Samanta, Evgeny Y. Tsymbal

Abstract: Spin-split antiferromagnets have significance for antiferromagnetic (AFM) spintronics due to their momentum dependent spin polarization which can be exploited for the control and detection of the AFM order parameter. Here, we explore the polar-layer stacking of AFM-ordered bilayers driving the emergence of reversable electric polarization and non-relativistic spin splitting (NRSS) of their band st… ▽ More Spin-split antiferromagnets have significance for antiferromagnetic (AFM) spintronics due to their momentum dependent spin polarization which can be exploited for the control and detection of the AFM order parameter. Here, we explore the polar-layer stacking of AFM-ordered bilayers driving the emergence of reversable electric polarization and non-relativistic spin splitting (NRSS) of their band structure. Based on the spin-space group approach, we identify several representative two-dimensional AFM materials which exhibit different types of NRSS when stacked into a polar bilayer. We demonstrate that NRSS can have both altermagnetic and non-altermagnetic origins and elucidate symmetry requirements for NRSS to be switchable by electric polarization. We argue that the electric polarization switching of NRSS in polar AFM bilayers may be more practical for device applications than the current-induced Néel vector switching. △ Less

Submitted 12 March, 2025; originally announced March 2025.

Comments: 8 pages, 4 figures

Coexistence of distinct mobility edges in a 1D quasiperiodic mosaic model

Authors: Xu Xia, Weihao Huang, Ke Huang, Xiaolong Deng, Xiao Li

Abstract: We introduce a one-dimensional quasiperiodic mosaic model with analytically solvable mobility edges that exhibit different phase transitions depending on the system parameters. Specifically, by combining mosaic quasiperiodic next-nearest-neighbor hoppings and quasiperiodic on-site potentials, we rigorously demonstrate the existence of two distinct types of mobility edges: those separating extended… ▽ More We introduce a one-dimensional quasiperiodic mosaic model with analytically solvable mobility edges that exhibit different phase transitions depending on the system parameters. Specifically, by combining mosaic quasiperiodic next-nearest-neighbor hoppings and quasiperiodic on-site potentials, we rigorously demonstrate the existence of two distinct types of mobility edges: those separating extended and critical states, and those separating extended and localized states. Using Avila's global theory, we derive exact analytical expressions for these mobility edges and determine the parameter regimes where each type dominates. Our numerical calculations confirm these analytical results through fractal dimension analysis. Furthermore, we propose an experimentally feasible scheme to realize this model using Bose-Einstein condensates in optical lattices with engineered momentum-state transitions. We also investigate the effects of many-body interactions under mean-field approximation. Our work provides a fertile ground for studying the coexistence of different types of mobility edges in quasiperiodic systems and suggests a feasible experimental platform to observe and control these transitions. △ Less

Submitted 6 March, 2025; originally announced March 2025.

Comments: 5 pages, 3 figures. Comments are welcome

Ultrafast annealing process of MTJ using hybrid microwave annealing

Authors: Ming-Chun Hsu, Fan-Yun Chiu, Wei-Chi Aeneas Hsu, Chang-Shan Shen, Kun-Ping Huang, Tsun-Hsu Chang

Abstract: This paper discovers that the magnetic tunnel junction (MTJ) structure is successfully magnetized with hybrid microwave annealing, confirmed by the tunneling magnetoresistance (TMR) and Coercivity (Hc) results. Hybrid microwave annealing can transform CoFeB into a single crystal and form the Fe-O bond at the interface between CoFeB and MgO without adding an extra magnet. The annealing time is sign… ▽ More This paper discovers that the magnetic tunnel junction (MTJ) structure is successfully magnetized with hybrid microwave annealing, confirmed by the tunneling magnetoresistance (TMR) and Coercivity (Hc) results. Hybrid microwave annealing can transform CoFeB into a single crystal and form the Fe-O bond at the interface between CoFeB and MgO without adding an extra magnet. The annealing time is significantly reduced from the original 120 minutes to just 1 minute, allowing for rapid low-temperature annealing of the MTJ structure. The TEM results are used to determine the change in the lattice structure of CoFeB from amorphous to a single crystal, and the EELS result indicates the diffusion distribution of atoms in the MTJ structure. This hybrid annealing process can save a significant amount of fabrication time and is an energy-efficient alternative to the current fabrication process of MRAM. △ Less

Submitted 18 February, 2025; originally announced February 2025.

Comments: 3 pages, 5 figures

Diagonal Symmetrization of Neural Network Solvers for the Many-Electron Schrödinger Equation

Authors: Kevin Han Huang, Ni Zhan, Elif Ertekin, Peter Orbanz, Ryan P. Adams

Abstract: Incorporating group symmetries into neural networks has been a cornerstone of success in many AI-for-science applications. Diagonal groups of isometries, which describe the invariance under a simultaneous movement of multiple objects, arise naturally in many-body quantum problems. Despite their importance, diagonal groups have received relatively little attention, as they lack a natural choice of… ▽ More Incorporating group symmetries into neural networks has been a cornerstone of success in many AI-for-science applications. Diagonal groups of isometries, which describe the invariance under a simultaneous movement of multiple objects, arise naturally in many-body quantum problems. Despite their importance, diagonal groups have received relatively little attention, as they lack a natural choice of invariant maps except in special cases. We study different ways of incorporating diagonal invariance in neural network ansätze trained via variational Monte Carlo methods, and consider specifically data augmentation, group averaging and canonicalization. We show that, contrary to standard ML setups, in-training symmetrization destabilizes training and can lead to worse performance. Our theoretical and numerical results indicate that this unexpected behavior may arise from a unique computational-statistical tradeoff not found in standard ML analyses of symmetrization. Meanwhile, we demonstrate that post hoc averaging is less sensitive to such tradeoffs and emerges as a simple, flexible and effective method for improving neural network solvers. △ Less

Submitted 28 August, 2025; v1 submitted 7 February, 2025; originally announced February 2025.

Comments: ICML 2025 camera-ready version

Interaction-enhanced many-body localization in a 1D quasiperiodic model with long-range hopping

Authors: Haowei Fan, Ke Huang, Xiao Li

Abstract: We study the many-body localization (MBL) transition in an 1D exactly solvable system with long-range hopping and quasiperiodic on-site potential introduced in Phys. Rev. Lett. 131, 186303 (2023). Unlike other disorder or quasiperiodic model, an interaction-enhanced MBL happens in the moderate interaction regime, which is dubbed as the interaction-enhanced MBL. This counterintuitive phenomenon can… ▽ More We study the many-body localization (MBL) transition in an 1D exactly solvable system with long-range hopping and quasiperiodic on-site potential introduced in Phys. Rev. Lett. 131, 186303 (2023). Unlike other disorder or quasiperiodic model, an interaction-enhanced MBL happens in the moderate interaction regime, which is dubbed as the interaction-enhanced MBL. This counterintuitive phenomenon can be understood by noticing the fragility of the critical band lying at the bottom of the spectrum. The fragile band is localized by other localized states once the interaction is turned on. This mechanism can be verified by introducing a mean-field theory description which can derive highly excited states with high accuracy. The effectiveness of this mean-field theory is captured by the quasihole physics, validated by the particle entanglement spectra. △ Less

Submitted 8 January, 2025; originally announced January 2025.

Comments: 8 pages, 8 figures. Comments are welcome

Prediction of polarization vortices, charge modulation, flat bands, and moiré magnetism in twisted oxide bilayers

Authors: Naafis Ahnaf Shahed, Kartik Samanta, Mohamed Elekhtiar, Kai Huang, Chang-Beom Eom, Mark S. Rzchowski, Kirill D. Belashchenko, Evgeny Y. Tsymbal

Abstract: The recent surge of interest in moiré superlattices of twisted van der Waals compounds has spotlighted the emergence of unconventional superconductivity and novel electronic phases. However, the range of moiré phenomena can be dramatically expanded by incorporating complex oxide materials into twisted heterostructures. In this study, motivated by the recent breakthroughs in synthesis of free-stand… ▽ More The recent surge of interest in moiré superlattices of twisted van der Waals compounds has spotlighted the emergence of unconventional superconductivity and novel electronic phases. However, the range of moiré phenomena can be dramatically expanded by incorporating complex oxide materials into twisted heterostructures. In this study, motivated by the recent breakthroughs in synthesis of free-standing oxide membranes, we explore the emergent structural and electronic properties of twisted oxide bilayers. We focus on the classic perovskite oxide, SrTiO3, and design SrTiO3 bilayers with a relative twist between the individual layers. Using density functional theory calculations, we predict the appearance of vortex-antivortex polarization patterns at the interface of the SrTiO3 bilayers driven by twist. We also predict charge modulation of the interfacial Ti ions induced by varying local coordination which follow the moiré pattern. Furthermore, we forecast the emergence of flat bands at large twist angles and the associated localized electronic states with moiré-periodic charge density, originating from the interlayer bonding effects resulting in the formation of dangling bonds. Finally, we predict that hole doping induces unconventional d0 magnetism in otherwise nonmagnetic SrTiO3, driven by the exchange splitting of the high-density O-p bands and producing the spin density with moiré periodicity. These results demonstrate a broad landscape of emergent phenomena which may occur in moiré-engineered oxide heterostructures showing far-reaching perspectives of these material systems for further fundamental studies and potential applications. △ Less

Submitted 4 December, 2024; originally announced December 2024.

Binding memory of liquid molecules

Authors: Shiyi Qin, Zhi Yang, Huimin Liu, Xiaoli Wang, Shangguo Hou, Kai Huang

Abstract: Understanding the binding dynamics of liquid molecules is of fundamental importance in physical and life sciences. However, nanoscale fast dynamics pose great challenges for experimental characterization. Conventionally, the binding dynamics have been assumed to be memoryless. Here, we integrate large scale computer simulation, scaling theory, and real-time single particle tracking microscopy with… ▽ More Understanding the binding dynamics of liquid molecules is of fundamental importance in physical and life sciences. However, nanoscale fast dynamics pose great challenges for experimental characterization. Conventionally, the binding dynamics have been assumed to be memoryless. Here, we integrate large scale computer simulation, scaling theory, and real-time single particle tracking microscopy with high spatiotemporal precision to unveil a universal memory effect in the binding dynamics of liquid molecules. This binding memory can be quantified by a binding time autocorrelation function, whose power-law decay depends not only on the binding affinity, but also on the topological and materials properties of the surrounding environment. Context-dependent biomolecular binding memory is likely exploited by biological systems to regulate biochemical reactions and biophysical processes. Deciphering this binding memory offers a novel strategy to probe complex biological systems and advanced soft materials. △ Less

Submitted 25 October, 2024; originally announced October 2024.

Nonlocal phase-change metaoptics for reconfigurable nonvolatile image processing

Authors: Guoce Yang, Mengyun Wang, June Sang Lee, Nikolaos Farmakidis, Joe Shields, Carlota Ruiz de Galarreta, Stuart Kendall, Jacopo Bertolotti, Andriy Moskalenko, Kairan Huang, Andrea Alù, C. David Wright, Harish Bhaskaran

Abstract: The next generation of smart imaging and vision systems will require compact and tunable optical computing hardware to perform high-speed and low-power image processing. These requirements are driving the development of computing metasurfaces to realize efficient front-end analog optical pre-processors, especially for edge-detection capability. Yet, there is still a lack of reconfigurable or progr… ▽ More The next generation of smart imaging and vision systems will require compact and tunable optical computing hardware to perform high-speed and low-power image processing. These requirements are driving the development of computing metasurfaces to realize efficient front-end analog optical pre-processors, especially for edge-detection capability. Yet, there is still a lack of reconfigurable or programmable schemes, which may drastically enhance the impact of these devices at the system level. Here, we propose and experimentally demonstrate a reconfigurable flat optical image processor using low-loss phase-change nonlocal metasurfaces. The metasurface is configured to realize different transfer functions in spatial frequency space, when transitioning the phase-change material between its amorphous and crystalline phases. This enables edge detection and bright-field imaging modes on the same device. The metasurface is compatible with a large numerical aperture of ~0.5, making it suitable for high resolution coherent optical imaging microscopy. The concept of phase-change reconfigurable nonlocal metasurfaces may enable emerging applications of artificial intelligence-assisted imaging and vision devices with switchable multitasking. △ Less

Submitted 17 September, 2024; originally announced September 2024.

Comments: 20 pages, 5 figures

Journal ref: Light Sci Appl 14, 182 (2025)

Impurity-induced thermal crossover in fractional Chern insulators

Authors: Ke Huang, Sankar Das Sarma, Xiao Li

Abstract: The recent experimental observation of fractional quantum anomalous Hall (FQAH) states in rhombohedral multilayer graphene has attracted significant attention. One of the most intriguing observations is that the FQAH states at various fractional fillings give way to IQAH states as the temperature is lowered. In this work, we propose a mechanism for the appearance of FQAH states within a finite tem… ▽ More The recent experimental observation of fractional quantum anomalous Hall (FQAH) states in rhombohedral multilayer graphene has attracted significant attention. One of the most intriguing observations is that the FQAH states at various fractional fillings give way to IQAH states as the temperature is lowered. In this work, we propose a mechanism for the appearance of FQAH states within a finite temperature range in a toy model. The model consists of a flat Chern band and impurities, and we analyze the effects of impurities on the system's behavior at finite temperatures. We believe that the crossover may arise from the competition between the energy penalty for thermal excitations and the increase in entropy. We support our theoretical argument with numerical calculations using exact diagonalization. Our results suggest that impurities may play a crucial role in the crossover from the FQAH to IQAH states in rhombohedral pentalayer graphene. △ Less

Submitted 6 September, 2024; originally announced September 2024.

Comments: 5 pages, 4 figures. Comments are welcome

Non-Abelian fractional quantum Hall states at filling factor 3/4

Authors: Kai-Wen Huang, Ying-Hai Wu

Abstract: Fractional quantum Hall states have been observed at filling factor $ν=3/4$ in three platforms. General theoretical analysis of topological orders at $ν=3/4$ revealed that four types of non-Abelian states with Ising anyons have ground state degeneracy $12$ on the torus. The properties of $ν=3/4$ states can be analyzed using two complementary approaches. In the first one, they are treated as partic… ▽ More Fractional quantum Hall states have been observed at filling factor $ν=3/4$ in three platforms. General theoretical analysis of topological orders at $ν=3/4$ revealed that four types of non-Abelian states with Ising anyons have ground state degeneracy $12$ on the torus. The properties of $ν=3/4$ states can be analyzed using two complementary approaches. In the first one, they are treated as particle-hole conjugate of $ν=1/4$ Moore-Read types states. In the second one, they are mapped to composite fermions with reverse flux attachment at effective filling factor $3/2$, whose integral part realizes an integer quantum Hall state and the fractional part realizes $ν=1/2$ Moore-Read type states. For the specific case of bilayer graphene, numerical calculations demonstrate that strong Landau level mixing could generate a gapped state at $ν=3/4$ with 12 fold ground state degeneracy on the torus. Its chiral graviton spectral functions has one low energy peak with negative chirality and one high energy peak with positive chirality. This points to a specific member of the Moore-Read type states and agrees with the deduction based on daughter states. △ Less

Submitted 29 August, 2024; originally announced August 2024.

Comments: 6 pages, 2 figures

Two-dimensional non-volatile valley spin valve

Authors: Kai Huang, Kartik Samanta, Ding-Fu Shao, Evgeny Y. Tsymbal

Abstract: A spin valve represents a well-established device concept in magnetic memory technologies, whose functionality is determined by electron transmission being controlled by the relative alignment of magnetic moments of the two ferromagnetic layers. Recently, the advent of valleytronics has conceptualized a valley spin valve (VSV) - a device that utilizes the valley degree of freedom and spin-valley l… ▽ More A spin valve represents a well-established device concept in magnetic memory technologies, whose functionality is determined by electron transmission being controlled by the relative alignment of magnetic moments of the two ferromagnetic layers. Recently, the advent of valleytronics has conceptualized a valley spin valve (VSV) - a device that utilizes the valley degree of freedom and spin-valley locking to achieve a similar valve effect without relying on magnetism. In this study, we propose a non-volatile VSV (n-VSV) based on a two-dimensional (2D) ferroelectric semiconductor where the resistance of the n-VSV is controlled by the ferroelectric domain wall between the two uniformly polarized domains. Focusing on the 1T'' phase of MoS2, which is known to be ferroelectric down to a monolayer and using density functional theory (DFT) combined with the quantum-transport calculations, we demonstrate that switching between the uniformly polarized state and the state with oppositely polarized domains separated by a domain wall results in resistance change of as high as 10^7. This giant VSV effect occurs due to transmission being strongly dependent on matching (mismatching) the valley-dependent spin polarizations in the two domains with the same (opposite) ferroelectric polarization orientations, when the chemical potential of 1T''-MoS2 lies within the spin-split valleys. Our work paves a new route for realizing high-performance nonvolatile valleytronics. △ Less

Submitted 21 August, 2024; originally announced August 2024.

Comments: 6 pages, 4 figures

High-Temperature Quantum Valley Hall Effect with Quantized Resistance and a Topological Switch

Authors: Ke Huang, Hailong Fu, Kenji Watanabe, Takashi Taniguchi, Jun Zhu

Abstract: Edge states of a topological insulator can be used to explore fundamental science emerging at the interface of low dimensionality and topology. Achieving a robust conductance quantization, however, has proven challenging for helical edge states. Here we show wide resistance plateaus in kink states - a manifestation of the quantum valley Hall effect in Bernal bilayer graphene - quantized to the pre… ▽ More Edge states of a topological insulator can be used to explore fundamental science emerging at the interface of low dimensionality and topology. Achieving a robust conductance quantization, however, has proven challenging for helical edge states. Here we show wide resistance plateaus in kink states - a manifestation of the quantum valley Hall effect in Bernal bilayer graphene - quantized to the predicted value at zero magnetic field. The plateau resistance has a very weak temperature dependence up to 50 Kelvin and is flat within a dc bias window of tens of mV. We demonstrate the electrical operation of a topology-controlled switch with an on/off ratio of 200. These results demonstrate the robustness and tunability of the kink states and its promise in constructing electron quantum optics devices. △ Less

Submitted 14 August, 2024; originally announced August 2024.

Comments: 21 pages, 5 main figures, with Supplementary Materials

Journal ref: Science 385, 657-661 (2024)

Tunable Second-Order Structural Transition in As-Deficient MnAs

Authors: B. D. White, K. Huang, I. L. Fipps, J. J. Hamlin, S. Jang, G. J. Smith, B. Xia, J. W. Simonson, C. S. Nelson, M. C. Aronson, M. B. Maple

Abstract: We report measurements of magnetization, specific heat, and thermal expansion performed on As-deficient MnAs single crystals (MnAs$_{0.968}$). Ferromagnetic order is observed near $T_C \simeq$ 306 K on warming and $T_C \simeq$ 302 K on cooling, which is consistent with previously-reported values for stoichiometric MnAs samples. In contrast, the second-order structural phase transition is observed… ▽ More We report measurements of magnetization, specific heat, and thermal expansion performed on As-deficient MnAs single crystals (MnAs$_{0.968}$). Ferromagnetic order is observed near $T_C \simeq$ 306 K on warming and $T_C \simeq$ 302 K on cooling, which is consistent with previously-reported values for stoichiometric MnAs samples. In contrast, the second-order structural phase transition is observed at $T_S \simeq$ 353 K, which is nearly 50 K lower than in the stoichiometric compound. We observe differences in the thermal expansion of our samples when compared to reports of stoichiometric MnAs including: (1) the $\sim$1.5% volume decrease at $T_C$ is smaller than the expected value of 1.9%, (2) the lattice parameters perpendicular to the basal plane exhibit a discontinuous jump of $\sim$1.1% at $T_C$ instead of being continuous across $T_C$, and (3) thermal expansion perpendicular to the basal plane for $T_C \le T \le$ 315 K is negative rather than positive. We also observe a correlation between the ratio of hexagonal lattice parameters, $c/a$, and $T_S$, strongly suggesting that the degree of structural anisotropy in MnAs could play an important role in tuning $T_S$. △ Less

Submitted 14 August, 2024; originally announced August 2024.

$^{19}$F NMR and defect spins in vacuum-annealed LaO$_{0.5}$F$_{0.5}$BiS$_2$

Authors: S. Yadav, S. Delgado, O. O. Bernal, D. E. MacLaughlin, Y. Liu, D. Jiang, O. Santana, A. Mushammel, Lei Shu, K. Huang, D. Yazici, M. B. Maple

Abstract: We report results of magnetization and $^{19}$F NMR measurements in the normal state of as-grown LaO$_{0.5}$F$_{0.5}$BiS$_2$. The magnetization is dominated by a temperature-independent diamagnetic component and a field- and temperature-dependent paramagnetic contribution $M_μ(H,T)$ from a $\sim$1000~ppm concentration of local moments, an order of magnitude higher than can be accounted for by meas… ▽ More We report results of magnetization and $^{19}$F NMR measurements in the normal state of as-grown LaO$_{0.5}$F$_{0.5}$BiS$_2$. The magnetization is dominated by a temperature-independent diamagnetic component and a field- and temperature-dependent paramagnetic contribution $M_μ(H,T)$ from a $\sim$1000~ppm concentration of local moments, an order of magnitude higher than can be accounted for by measured rare-earth impurity concentrations. $M_μ(H,T)$ can be fit by the Brillouin function $B_J(x)$ or, perhaps more realistically, a two-level $\tanh(x)$ model for magnetic Bi $6p$ ions in defect crystal fields. Both fits require a phenomenological Curie-Weiss argument $x = μ_\mathrm{eff}H/(T + T_W)$, $T_W \approx 1.7$ K. There is no evidence for magnetic order down to 2 K, and the origin of $T_W$ is not clear. $^{19}$F frequency shifts, linewidths, and spin-lattice relaxation rates are consistent with purely dipolar $^{19}$F/defect-spin interactions. The defect-spin correlation time $τ_c(T)$ obtained from $^{19}$F spin-lattice relaxation rates obeys the Korringa relation $τ_cT = \text{const.}$, indicating the relaxation is dominated by conduction-band fluctuations. △ Less

Submitted 13 August, 2024; v1 submitted 12 August, 2024; originally announced August 2024.

Comments: Corrected citations, 15 pages, 15 figures, 6 tables, 69 references, corresponding author O. O. Bernal: obernal@calstatela.edu

Fractional quantum anomalous Hall effect in rhombohedral multilayer graphene with a strong displacement field

Authors: Ke Huang, Sankar Das Sarma, Xiao Li

Abstract: We investigate the fractional quantum anomalous Hall (FQAH) effect in rhombohedral multilayer graphene (RnG) in the presence of a strong applied displacement field. We first introduce the interacting model of RnG, which includes the noninteracting continuum model and the many-body Coulomb interaction. We then discuss the integer quantum anomalous Hall (IQAH) effect in RnG and the role of the Hartr… ▽ More We investigate the fractional quantum anomalous Hall (FQAH) effect in rhombohedral multilayer graphene (RnG) in the presence of a strong applied displacement field. We first introduce the interacting model of RnG, which includes the noninteracting continuum model and the many-body Coulomb interaction. We then discuss the integer quantum anomalous Hall (IQAH) effect in RnG and the role of the Hartree-Fock approach in understanding its appearance. Next, we explore the FQAH effect in RnG for $n=3$--$6$ using a combination of constrained Hartree-Fock and exact diagonalization methods. We characterize the stability of the FQAH phase by the size of the FQAH gap and find that RnG generally has a stable FQAH phase, although the required displacement field varies significantly among different $n$ values. Our work establishes the theoretical universality of both IQAH and FQAH in RnG. △ Less

Submitted 14 February, 2025; v1 submitted 9 August, 2024; originally announced August 2024.

Comments: 7 pages, 6 figures. Comments are welcome

Journal ref: Phys. Rev. B 111, 075130 (2025)

Antiferroelectric Hafnia Down to the 2D Limit

Authors: Xin Li, Guodong Ren, Haidong Lu, Kartik Samanta, Amit Kumar Shah, Kai Huang, Pravan Omprakash, Yu Yun, Pratyush Buragohain, Huibo Cao, Yan Wu, Jordan A. Hachtel, Andrew R. Lupini, Miaofang Chi, Juan Carlos Idrobo, Evgeny Y. Tsymbal, Alexei Gruverman, Rohan Mishra, Xiaoshan Xu

Abstract: Antiferroelectricity is a material property characterized by alternating electric dipoles spontaneously ordered in antiparallel directions. Antiferroelectrics are promising for energy storage, solid-state cooling, and memory technologies; however, these materials are scarce, and their scalability remains largely unexplored. In this work, we demonstrate that single-crystalline hafnia, a lead-free C… ▽ More Antiferroelectricity is a material property characterized by alternating electric dipoles spontaneously ordered in antiparallel directions. Antiferroelectrics are promising for energy storage, solid-state cooling, and memory technologies; however, these materials are scarce, and their scalability remains largely unexplored. In this work, we demonstrate that single-crystalline hafnia, a lead-free CMOS-compatible material, exhibits antiferroelectricity under compressive-strain conditions. We observe antiparallel sublattice polarization and stable double-hysteresis in single-crystalline (111)-oriented epitaxial La-doped hafnia films grown on yttrium-stabilized zirconia and show that the antipolar orthorhombic phase of hafnia adheres to the Kittel model of antiferroelectricity. Notably, compressive strain strengthens the antiferroelectric order in thinner La-doped hafnia films, achieving an unprecedented 850 C ordering temperature in the two-dimensional limit, highlighting hafnia's potential for advanced antiferroelectric devices. △ Less

Submitted 2 August, 2025; v1 submitted 3 August, 2024; originally announced August 2024.

First-principles investigation of the emergence of multiferroicity and skyrmions in CrI2

Authors: Khimananda Acharya, Kai Huang, Evgeny Y. Tsymbal, Tula R. Paudel

Abstract: The recently discovered van der Waals magnetic semiconductor CrI2 shows promise for spintronic applications. Its electronic and magnetic properties are known to be strongly influenced by electronic correlations. In this work, we employ density functional theory calculations where electronic correlations in CrI2 are considered within an on-site Coulomb interaction, U, with U being determined using… ▽ More The recently discovered van der Waals magnetic semiconductor CrI2 shows promise for spintronic applications. Its electronic and magnetic properties are known to be strongly influenced by electronic correlations. In this work, we employ density functional theory calculations where electronic correlations in CrI2 are considered within an on-site Coulomb interaction, U, with U being determined using the perturbative approach based on the linear response method. We show that the accuracy of an on-site Coulomb interaction is essential for predicting the non-centrosymmetric orthorhombic ground state of CrI2, which allows the existence of ferroelectricity and non-zero Dzyaloshinskii-Moriya (DMI) interaction. Our calculation shows that the ground state of bulk CrI2 has electric polarization of 0.15 μC/cm-2 pointing along the z-axis and the small DMI energy that changes sign with the ferroelectric polarization switching. The DMI and polarization are enhanced to 0.28 meV/formula-unit and 0.63 μC/cm-2 when Pt intercalates bilayers of CrI2, due to its large spin-orbit coupling strength and large off-center displacement. Such an enhanced DMI leads to the Néel skyrmions, whose handedness is controlled by ferroelectric polarization. Our work contributes to the creation and manipulation of bits in skyrmions-based memory devices. △ Less

Submitted 2 August, 2024; originally announced August 2024.

Self-consistent theory for the fractional quantum anomalous Hall effect in rhombohedral pentalayer graphene

Authors: Ke Huang, Xiao Li, Sankar Das Sarma, Fan Zhang

Abstract: The fractional quantum anomalous Hall (FQAH) effect in rhombohedral pentalayer graphene (PLG) has attracted significant attention due to its potential for observing exotic quantum states. In this work, we present a self-consistent Hartree-Fock theory for the FQAH effect in rhombohedral PLG. In particular, we focus on the convergence of the Hartree-Fock calculation with various reference fields and… ▽ More The fractional quantum anomalous Hall (FQAH) effect in rhombohedral pentalayer graphene (PLG) has attracted significant attention due to its potential for observing exotic quantum states. In this work, we present a self-consistent Hartree-Fock theory for the FQAH effect in rhombohedral PLG. In particular, we focus on the convergence of the Hartree-Fock calculation with various reference fields and discuss the stability of the FQAH states in PLG. We show that the so-called charge neutrality scheme provides an unambiguous result for the Hartree-Fock calculation, as it ensures a convergence with respect to the momentum cutoff. Based on the Hartree-Fock band structure, we further carry out exact diagonalization calculations to explore the stability of the FQAH states in PLG. Our work provides an improved and unified (minimal) theoretical framework to understand the FQAH effect in rhombohedral PLG and paves the way for future experimental and theoretical studies. △ Less

Submitted 17 October, 2024; v1 submitted 11 July, 2024; originally announced July 2024.

Comments: 19 pages, 12 figures. Comments are welcome

Journal ref: Phys. Rev. B 110, 115146 (2024)

Insulator-to-Metal Transition and Anomalously Slow Hot Carrier Cooling in a Photo-doped Mott Insulator

Authors: Usama Choudhry, Jin Zhang, Kewen Huang, Emma Low, Yujie Quan, Basamat Shaheen, Ryan Gnabasik, Jiaqiang Yan, Angel Rubio, Kenneth S. Burch, Bolin Liao

Abstract: Photo-doped Mott insulators can exhibit novel photocarrier transport and relaxation dynamics and non-equilibrium phases. However, time-resolved real-space imaging of these processes are still lacking. Here, we use scanning ultrafast electron microscopy (SUEM) to directly visualize the spatial-temporal evolution of photoexcited species in a spin-orbit assisted Mott insulator α-RuCl3. At low optical… ▽ More Photo-doped Mott insulators can exhibit novel photocarrier transport and relaxation dynamics and non-equilibrium phases. However, time-resolved real-space imaging of these processes are still lacking. Here, we use scanning ultrafast electron microscopy (SUEM) to directly visualize the spatial-temporal evolution of photoexcited species in a spin-orbit assisted Mott insulator α-RuCl3. At low optical fluences, we observe extremely long hot photocarrier transport time over one nanosecond, almost an order of magnitude longer than any known values in conventional semiconductors. At higher optical fluences, we observe nonlinear features suggesting a photo-induced insulator-to-metal transition, which is unusual in a large-gap Mott insulator. Our results demonstrate the rich physics in a photo-doped Mott insulator that can be extracted from spatial-temporal imaging and showcase the capability of SUEM to sensitively probe photoexcitations in strongly correlated electron systems. △ Less

Submitted 11 June, 2024; originally announced June 2024.

Comments: Comments are welcome. Please email feedback to bliao@ucsb.edu

Exploring Hilbert-Space Fragmentation on a Superconducting Processor

Authors: Yong-Yi Wang, Yun-Hao Shi, Zheng-Hang Sun, Chi-Tong Chen, Zheng-An Wang, Kui Zhao, Hao-Tian Liu, Wei-Guo Ma, Ziting Wang, Hao Li, Jia-Chi Zhang, Yu Liu, Cheng-Lin Deng, Tian-Ming Li, Yang He, Zheng-He Liu, Zhen-Yu Peng, Xiaohui Song, Guangming Xue, Haifeng Yu, Kaixuan Huang, Zhongcheng Xiang, Dongning Zheng, Kai Xu, Heng Fan

Abstract: Isolated interacting quantum systems generally thermalize, yet there are several counterexamples for the breakdown of ergodicity, such as many-body localization and quantum scars. Recently, ergodicity breaking has been observed in systems subjected to linear potentials, termed Stark many-body localization. This phenomenon is closely associated with Hilbert-space fragmentation, characterized by a s… ▽ More Isolated interacting quantum systems generally thermalize, yet there are several counterexamples for the breakdown of ergodicity, such as many-body localization and quantum scars. Recently, ergodicity breaking has been observed in systems subjected to linear potentials, termed Stark many-body localization. This phenomenon is closely associated with Hilbert-space fragmentation, characterized by a strong dependence of dynamics on initial conditions. Here, we experimentally explore initial-state dependent dynamics using a ladder-type superconducting processor with up to 24 qubits, which enables precise control of the qubit frequency and initial state preparation. In systems with linear potentials, we observe distinct non-equilibrium dynamics for initial states with the same quantum numbers and energy, but with varying domain wall numbers. This distinction becomes increasingly pronounced as the system size grows, in contrast with disordered interacting systems. Our results provide convincing experimental evidence of the fragmentation in Stark systems, enriching our understanding of the weak breakdown of ergodicity. △ Less

Submitted 14 March, 2024; originally announced March 2024.

Comments: main text: 7 pages, 4 figures; supplementary: 13 pages, 14 figures

Journal ref: PRX Quantum 6, 010325 (2025)

Pseudospin Polarization of Composite Fermions under Uniaxial Strain

Authors: Shuai Yuan, Jiaojie Yan, Ke Huang, Zhimou Chen, Haoran Fan, L. N. Pfeiffer, K. W. West, Yang Liu, Xi Lin

Abstract: A two dimensional system with extra degrees of freedom, such as spin and valley, is of great interest in the study of quantum phase transitions. The critical condition when a transition between different multicomponent fractional quantum Hall states appears is one of the very few junctions for many body problems between theoretical calculations and experiments. In this work, we present that uniaxi… ▽ More A two dimensional system with extra degrees of freedom, such as spin and valley, is of great interest in the study of quantum phase transitions. The critical condition when a transition between different multicomponent fractional quantum Hall states appears is one of the very few junctions for many body problems between theoretical calculations and experiments. In this work, we present that uniaxial strain induces pseudospin transitions of composite fermions in a two-dimensional hole gas. Determined from transport behavior, strain along <111> effectively changes pseudospin energy levels. We deduce that diagonal strain dominates these variations. Our experiment provides a wedge for manipulating two dimensional interacting systems mechanically. △ Less

Submitted 6 March, 2024; originally announced March 2024.

Comments: 12 pages with 4 figures and 9 pages with 4 figures

Journal ref: Phys. Rev. B 109, L081110 (2024)

Bulk synthesis of Zn$_3$WN$_4$ via solid-state metathesis

Authors: Christopher L. Rom, Shaun O'Donnell, Kayla Huang, Ryan A. Klein, Morgan J. Kramer, Rebecca W. Smaha, Andriy Zakutayev

Abstract: Ternary nitrides are of growing technological importance, with applications as semiconductors, catalysts, and magnetic materials; however, new synthetic tools are needed to advance materials discovery efforts. Here, we show that Zn$_3$WN$_4$ can be synthesized via metathesis reactions between Li$_6$WN$_4$ and Zn$X_2$ ($X$ = Br, Cl, F). In situ synchrotron powder X-ray diffraction and differential… ▽ More Ternary nitrides are of growing technological importance, with applications as semiconductors, catalysts, and magnetic materials; however, new synthetic tools are needed to advance materials discovery efforts. Here, we show that Zn$_3$WN$_4$ can be synthesized via metathesis reactions between Li$_6$WN$_4$ and Zn$X_2$ ($X$ = Br, Cl, F). In situ synchrotron powder X-ray diffraction and differential scanning calorimetry show that the reaction onset is correlated with the Zn$X_2$ melting point and that product purity is inversely correlated with the reaction's exothermicity. High resolution synchrotron powder X-ray diffraction measurements show that this bulk synthesis produces a structure with substantial cation ordering, as opposed to the disordered structure initially discovered via thin film sputtering. Diffuse reflectance spectroscopy reveals that Zn$_3$WN$_4$ powders exhibit two optical absorption onsets at 2.5 eV and 4.0 eV, indicating wide-bandgap semiconducting behavior and suggesting a small amount of structural disorder. We hypothesize that this synthesis strategy is generalizable because many potential Li-$M$-N precursors (where $M$ is a metal) are available for synthesizing new ternary nitride materials. This work introduces a promising synthesis strategy that will accelerate the discovery of novel functional ternary nitrides and other currently inaccessible materials. △ Less

Submitted 5 January, 2024; v1 submitted 3 January, 2024; originally announced January 2024.

Journal ref: Chem. Sci. 2024

Probing spin hydrodynamics on a superconducting quantum simulator

Authors: Yun-Hao Shi, Zheng-Hang Sun, Yong-Yi Wang, Zheng-An Wang, Yu-Ran Zhang, Wei-Guo Ma, Hao-Tian Liu, Kui Zhao, Jia-Cheng Song, Gui-Han Liang, Zheng-Yang Mei, Jia-Chi Zhang, Hao Li, Chi-Tong Chen, Xiaohui Song, Jieci Wang, Guangming Xue, Haifeng Yu, Kaixuan Huang, Zhongcheng Xiang, Kai Xu, Dongning Zheng, Heng Fan

Abstract: Characterizing the nature of hydrodynamical transport properties in quantum dynamics provides valuable insights into the fundamental understanding of exotic non-equilibrium phases of matter. Experimentally simulating infinite-temperature transport on large-scale complex quantum systems is of considerable interest. Here, using a controllable and coherent superconducting quantum simulator, we experi… ▽ More Characterizing the nature of hydrodynamical transport properties in quantum dynamics provides valuable insights into the fundamental understanding of exotic non-equilibrium phases of matter. Experimentally simulating infinite-temperature transport on large-scale complex quantum systems is of considerable interest. Here, using a controllable and coherent superconducting quantum simulator, we experimentally realize the analog quantum circuit, which can efficiently prepare the Haar-random states, and probe spin transport at infinite temperature. We observe diffusive spin transport during the unitary evolution of the ladder-type quantum simulator with ergodic dynamics. Moreover, we explore the transport properties of the systems subjected to strong disorder or a tilted potential, revealing signatures of anomalous subdiffusion in accompany with the breakdown of thermalization. Our work demonstrates a scalable method of probing infinite-temperature spin transport on analog quantum simulators, which paves the way to study other intriguing out-of-equilibrium phenomena from the perspective of transport. △ Less

Submitted 1 September, 2024; v1 submitted 10 October, 2023; originally announced October 2023.

Comments: Main text: 13 pages, 5 figures; Supplementary: 17 pages, 16 figures, 1 table

Journal ref: Nat. Commun. 15, 7573 (2024)

Competition of electronic correlation and reconstruction in La1-xSrxTiO3/SrTiO3 heterostructures

Authors: Xueyan Wang, Lin Sun, Chen Ye, Zhen Huang, Kun Han, Ke Huang, Allen Jian Yang, Shengwei Zeng, Xian Jun Loh, Qiang Zhu, T. Venkatesan, Ariando Ariando, X. Renshaw Wang

Abstract: Electronic correlation and reconstruction are two important factors that play a critical role in shaping the magnetic and electronic properties of correlated low-dimensional systems. Here, we report a competition between the electronic correlation and structural reconstruction in La1-xSrxTiO3/SrTiO3 heterostructures by modulating material polarity and interfacial strain, respectively. The heterost… ▽ More Electronic correlation and reconstruction are two important factors that play a critical role in shaping the magnetic and electronic properties of correlated low-dimensional systems. Here, we report a competition between the electronic correlation and structural reconstruction in La1-xSrxTiO3/SrTiO3 heterostructures by modulating material polarity and interfacial strain, respectively. The heterostructures exhibit a critical thickness (tc) at which a metal-to-insulator transition (MIT) abruptly occurs at certain thickness, accompanied by the coexistence of two- and three-dimensional (2D and 3D) carriers. Intriguingly, the tc exhibits a V-shaped dependence on the doping concentration of Sr, with the smallest tc value at x = 0.5. We attribute this V-shaped dependence to the competition between the electronic reconstruction (modulated by the polarity) and the electronic correlation (modulated by strain), which are borne out by the experimental results, including strain-dependent electronic properties and the evolution of 2D and 3D carriers. Our findings underscore the significance of the interplay between electronic reconstruction and correlation in the realization and utilization of emergent electronic functionalities in low-dimensional correlated systems. △ Less

Submitted 6 October, 2023; originally announced October 2023.

Out-of-time-order correlator, many-body quantum chaos, light-like generators, and singular values

Authors: Ke Huang, Xiao Li, David A. Huse, Amos Chan

Abstract: We study out-of-time-order correlators (OTOCs) of local operators in spatial-temporal invariant or random quantum circuits using light-like generators (LLG) -- many-body operators that exist in and act along the light-like directions. We demonstrate that the OTOC can be approximated by the leading singular value of the LLG, which, for the case of generic many-body chaotic circuits, is increasingly… ▽ More We study out-of-time-order correlators (OTOCs) of local operators in spatial-temporal invariant or random quantum circuits using light-like generators (LLG) -- many-body operators that exist in and act along the light-like directions. We demonstrate that the OTOC can be approximated by the leading singular value of the LLG, which, for the case of generic many-body chaotic circuits, is increasingly accurate as the size of the LLG, $w$, increases. We analytically show that the OTOC has a decay with a universal form in the light-like direction near the causal light cone, as dictated by the sub-leading eigenvalues of LLG, $z_2$, and their degeneracies. Further, we analytically derive and numerically verify that the sub-leading eigenvalues of LLG of any size can be accessibly extracted from those of LLG of the smallest size, i.e., $z_2(w)= z_2(w=1)$. Using symmetries and recursive structures of LLG, we propose two conjectures on the universal aspects of generic many-body quantum chaotic circuits, one on the algebraic degeneracy of eigenvalues of LLG, and another on the geometric degeneracy of the sub-leading eigenvalues of LLG. As corollaries of the conjectures, we analytically derive the asymptotic form of the leading singular state, which in turn allows us to postulate and efficiently compute a product-state variational ansatz away from the asymptotic limit. We numerically test the claims with four generic circuit models of many-body quantum chaos, and contrast these statements against the cases of a dual unitary system and an integrable system. △ Less

Submitted 9 October, 2023; v1 submitted 30 August, 2023; originally announced August 2023.

Comments: 6 + 15 pages, 3 + 11 figures. Comments are welcome. Updated on 2023-10-10

Magnetic Antiskyrmions in Two-Dimensional van der Waals Magnets Engineered by Layer Stacking

Authors: Kai Huang, Edward Schwartz, Ding-Fu Shao, Alexey A. Kovalev, Evgeny Y. Tsymbal

Abstract: Magnetic skyrmions and antiskyrmions are topologically protected quasiparticles exhibiting a whirling spin texture in real space. Antiskyrmions offer some advantages over skyrmions as they are expected to have higher stability and can be electrically driven with no transverse motion. However, unlike the widely investigated skyrmions, antiskyrmions are rarely observed due to the required anisotropi… ▽ More Magnetic skyrmions and antiskyrmions are topologically protected quasiparticles exhibiting a whirling spin texture in real space. Antiskyrmions offer some advantages over skyrmions as they are expected to have higher stability and can be electrically driven with no transverse motion. However, unlike the widely investigated skyrmions, antiskyrmions are rarely observed due to the required anisotropic Dzyaloshinskii-Moriya interaction (DMI). Here we propose to exploit the recently demonstrated van der Waals (vdW) assembly of two-dimensional (2D) materials that breaks inversion symmetry and creates conditions for anisotropic DMI. Using a 2D vdW magnet CrI${}_3$ as an example, we demonstrate, based on density functional theory (DFT) calculations, that this strategy is a promising platform to realize antiskyrmions. Polar layer stacking of two centrosymmetric magnetic monolayers of CrI${}_3$ efficiently lowers the symmetry, resulting in anisotropic DMI that supports antiskyrmions. The DMI is reversible by switching the ferroelectric polarization inherited from the polar layer stacking, offering the control of antiskyrmions by an electric field. Furthermore, we find that the magnetocrystalline anisotropy and DMI of CrI${}_3$ can be efficiently modulated by Mn doping, creating a possibility to control the size of antiskyrmions. Using atomistic spin dynamics simulations with the parameters obtained from our DFT calculations, we predict the formation of antiskyrmions in a Cr${}_{0.88}$Mn${}_{0.12}$I${}_3$ bilayer and switching their spin texture with polarization reversal. Our results open a new direction to generate and control magnetic antiskyrmions in 2D vdW magnetic systems. △ Less

Submitted 28 July, 2023; originally announced July 2023.

Comments: 10 pages, 6 figures

Statistics of noninteracting many-body fermionic states: The question of a many-body mobility edge

Authors: Ke Huang, DinhDuy Vu, Sankar Das Sarma, Xiao Li

Abstract: In this work, we study the statistics of a generic noninteracting many-body fermionic system whose single-particle counterpart has a single-particle mobility edge (SPME). We first prove that the spectrum and the extensive conserved quantities follow the multivariate normal distribution with a vanishing standard deviation $\sim O(1/\sqrt L)$ in the thermodynamic limit, regardless of SPME. Consequen… ▽ More In this work, we study the statistics of a generic noninteracting many-body fermionic system whose single-particle counterpart has a single-particle mobility edge (SPME). We first prove that the spectrum and the extensive conserved quantities follow the multivariate normal distribution with a vanishing standard deviation $\sim O(1/\sqrt L)$ in the thermodynamic limit, regardless of SPME. Consequently, the theorem rules out an infinite-temperature or high-temperature many-body mobility edge (MBME) for generic noninteracting fermionic systems. Further, we also prove that the spectrum of a fermionic many-body system with short-range interactions is qualitatively similar to that of a noninteracting many-body system up to the third-order moment. These results partially explain why neither short-range [1] nor long-range interacting systems exhibit an infinite-temperature MBME. △ Less

Submitted 20 June, 2023; originally announced June 2023.

Comments: 14 pages, 5 figures. Comments are welcome

Journal ref: Phys. Rev. B 109, 174214 (2024)

Interaction-enhanced many body localization in a generalized Aubry-Andre model

Authors: Ke Huang, DinhDuy Vu, Sankar Das Sarma, Xiao Li

Abstract: We study the many-body localization (MBL) transition in a generalized Aubry-Andre model (also known as the GPD model) introduced in Phys. Rev. Lett. 114, 146601 (2015). In contrast to MBL in other disordered or quasiperiodic models, the interaction seems to unexpectedly enhance MBL in the GPD model in some parameter ranges. To understand this counter-intuitive result, we demonstrate that the highe… ▽ More We study the many-body localization (MBL) transition in a generalized Aubry-Andre model (also known as the GPD model) introduced in Phys. Rev. Lett. 114, 146601 (2015). In contrast to MBL in other disordered or quasiperiodic models, the interaction seems to unexpectedly enhance MBL in the GPD model in some parameter ranges. To understand this counter-intuitive result, we demonstrate that the highest-energy single-particle band in the GPD model is unstable against even infinitesimal disorder, which leads to this surprising MBL phenomenon in the interacting model. We develop a mean-field theory description to understand the coupling between extended and localized states, which we validate using extensive exact diagonalization and DMRG-X numerical results. △ Less

Submitted 31 May, 2023; originally announced May 2023.

Comments: 5 pages and 5 figures. Comments are welcome

Journal ref: Phys. Rev. Res. 6, L022054 (2024)

Engineering subharmonic responses beyond prethermalization via Floquet scar states

Authors: Ke Huang, Xiao Li

Abstract: In this work we propose a new scheme to engineer subharmonic responses via scar states in a generalized PXP model. We first show that the generalized PXP model also possesses a band of scar states like the pristine PXP model does. In addition, we reveal that a generalized forward scattering approximation (FSA) still works for these scar states. We further argue that the FSA subspace exhibits an… ▽ More In this work we propose a new scheme to engineer subharmonic responses via scar states in a generalized PXP model. We first show that the generalized PXP model also possesses a band of scar states like the pristine PXP model does. In addition, we reveal that a generalized forward scattering approximation (FSA) still works for these scar states. We further argue that the FSA subspace exhibits an $SO(3)$ symmetry, which enables an $SO(3)$-FSA approach for the scar states. When such a model is placed under periodic driving, nontrivial Floquet scar states emerge in the quasienergy spectrum. One appealing feature of such Floquet scar states is that they can support subharmonic responses akin to the discrete time crystalline phase. In particular, such subharmonic responses can exist beyond the conventional prethermalization regime, where either large frequencies or large driving amplitudes are required. △ Less

Submitted 19 May, 2023; originally announced May 2023.

Comments: 5 pages, 4 figures. Comments are welcome

Journal ref: Phys. Rev. B 109, 064306 (2024)

Phase diagram of the $ν= 2$ quantum Hall state in bilayer graphene

Authors: Udit Khanna, Ke Huang, Ganpathy Murthy, H. A. Fertig, Kenji Watanabe, Takashi Taniguchi, Jun Zhu, Efrat Shimshoni

Abstract: Bilayer graphene exhibits a rich phase diagram in the quantum Hall regime, arising from a multitude of internal degrees of freedom, including spin, valley, and orbital indices. The variety of fractional quantum Hall states between filling factors $1 < ν\leq 2$ suggests, among other things, a quantum phase transition between valley-unpolarized and polarized states at a perpendicular electric field… ▽ More Bilayer graphene exhibits a rich phase diagram in the quantum Hall regime, arising from a multitude of internal degrees of freedom, including spin, valley, and orbital indices. The variety of fractional quantum Hall states between filling factors $1 < ν\leq 2$ suggests, among other things, a quantum phase transition between valley-unpolarized and polarized states at a perpendicular electric field $D^{*}$. We find the behavior of $D^{*}$ with $ν$ changes markedly as $B$ is reduced. At $ν= 2$, $D^{*}$ may even vanish when $B$ is sufficiently small. We present a theoretical model for lattice-scale interactions which explains these observations; surprisingly, both repulsive and attractive components in the interactions are required. Within this model we analyze the nature of the $ν= 2$ state as a function of the magnetic and electric fields, and predict that valley-coherence may emerge for $D \sim D^{*}$ in the high $B$ regime. This suggests the system supports Kekule bond-ordering, which could in principle be verified via STM measurements. △ Less

Submitted 5 May, 2023; originally announced May 2023.

Comments: 7 pages, 3 figures and Supplementary Material

Development of a Laser-based angle-resolved-photoemission spectrometer with sub-micrometer spatial resolution and high-efficiency spin detection

Authors: R. Z. Xu, X. Gu, W. X. Zhao, J. S. Zhou, Q. Q. Zhang, X. Du, Y. D. Li, Y. H. Mao, D. Zhao, K. Huang, C. F. Zhang, F. Wang, Z. K. Liu, Y. L. Chen, L. X. Yang

Abstract: Angle-resolved photoemission spectroscopy with sub-micrometer spatial resolution (μ-ARPES), has become a powerful tool for studying quantum materials. To achieve sub-micrometer or even nanometer-scale spatial resolution, it is important to focus the incident light beam (usually from the synchrotron radiation) using X-ray optics such as the zone plate or ellipsoidal capillary mirrors. Recently, we… ▽ More Angle-resolved photoemission spectroscopy with sub-micrometer spatial resolution (μ-ARPES), has become a powerful tool for studying quantum materials. To achieve sub-micrometer or even nanometer-scale spatial resolution, it is important to focus the incident light beam (usually from the synchrotron radiation) using X-ray optics such as the zone plate or ellipsoidal capillary mirrors. Recently, we developed a laser-based μ-ARPES with spin-resolution (LMS-ARPES). The 177 nm laser beam is achieved by frequency doubling a 355 nm beam using a KBBF crystal and subsequently focused using an optical lens with a focal length of about 16 mm. By characterizing the focused spot size using different methods and performing spatial-scanning photoemission measurement, we confirm the sub-micron spatial resolution of the system. Compared with the μ-ARPES facilities based on synchrotron radiation, our LMS-ARPES system is not only more economical and convenient but also with higher photon flux (> 5E13 photons/s), thus enabling the high-resolution and high-statistics measurements. Moreover, the system is equipped with a two-dimensional spin detector based on exchange scattering at a surface-passivated iron film grown on a W(100) substrate. We investigate the spin structure of the prototype topological insulator Bi2Se3 and reveal a high spin-polarization rate, confirming its spin-momentum locking property. This lab-based LMS-ARPES will be a powerful research tool for studying the local fine electronic structures of different condensed matter systems, including topological quantum materials, mesoscopic materials and structures, and phase-separated materials. △ Less

Submitted 30 January, 2023; originally announced January 2023.

Comments: To appear in RSI

Switchable anomalous Hall effects in polar-stacked 2D antiferromagnet MnBi2Te4

Authors: Tengfei Cao, Ding-Fu Shao, Kai Huang, Gautam Gurung, Evgeny Y. Tsymbal

Abstract: Van der Waals (vdW) assembly allows controlling symmetry of two-dimensional (2D) materials that determines their physical properties. Especially interesting is the recently demonstrated breaking inversion symmetry by polar layer stacking to realize novel electronic, magnetic, and transport properties of 2D vdW materials switchable by induced electric polarization. Here, based on symmetry analyses… ▽ More Van der Waals (vdW) assembly allows controlling symmetry of two-dimensional (2D) materials that determines their physical properties. Especially interesting is the recently demonstrated breaking inversion symmetry by polar layer stacking to realize novel electronic, magnetic, and transport properties of 2D vdW materials switchable by induced electric polarization. Here, based on symmetry analyses and density-functional calculations, we explore the emergence of the anomalous Hall effect (AHE) in antiferromagnetic MnBi2Te4 films assembled by polar layer stacking. We demonstrate that breaking PT symmetry in an MnBi2Te4 bilayer makes this 2D material magnetoelectric and produces a spontaneous AHE switchable by electric polarization. We find that reversable polarization at one of the interfaces in a three-layer MnBi2Te4 film drives a metal-insulator transition, as well as switching between an AHE and quantum AHE (QAHE). Finally, we predict that engineering an interlayer polarization in a three-layer MnBi2Te4 film allows converting MnBi2Te4 from a trivial insulator to a Chern insulator. Overall, our work emphasizes the emergence of quantum-transport phenomena in 2D vdW antiferromagnets by polar layer stacking, which do not exist in this material in the bulk or bulk-like thin-film forms. △ Less

Submitted 26 January, 2023; originally announced January 2023.